Coated paper has been the only material used in the past for thermal drop-on-demand printing, to overcome feathering. The feathering is particularly aggravated by the presence of the excessive (greater than 40 weight %) amounts of humectant needed to reduce "crusting-over" and relaxation times in the print head. As used herein, "thermal drop-on-demand printing" means printers that generate drops only when printing is needed, in response to thermal energy. Thus, for example, Hewlett-Packard's "Think Jet Black Ink" printed from drop-on-demand printers uses specially coated paper. It is not acceptable for use on plain paper, because the optical density is insufficient (no greater than about 0.75) when printed in a standard test pattern. This density, or lack of it, is not due to the amount of colorant present, since it can be shown that a greater amount of the colorant used by this ink gives no appreciable improvement in optical density.
The coatings needed for these specialty papers now used for drop-on-demand printing, are frequently calcium carbonate-based or silicate-based. In any event, such coatings usually result in paper costs that are two to five times as much as plain paper. The cost has not been due just to materials, but also to the process of achieving the coating. In addition to cost, the need to use specialty papers has been a substantial inconvenience, since the customer cannot use just any paper at hand. Furthermore, the recommended specialty papers are difficult to obtain in foreign countries.
Thus, ever since thermal drop-on-demand printing has been available, there has been a substantial need for an ink that does not require coated papers, but is useful on plain paper, even when the ink has greater than 40 weight % penetrant or humectant.
Surfactants have long been used with ink jet inks, for various purposes. Clearly, however, none of these have been found, heretofore, to be effective to render the thermal drop-on-demand ink printable on plain paper.
A typical example of surfactants used in inks is that shown in U.S. Pat. No. 4,184,881, wherein Surfynol 485, one of the acetylenic diols found useful herein, is listed as being useful as a humectant, "in the absence of any glycol". That patent is silent about using this surfactant to allow printing of drop-on-demand ink on plain paper. That is not surprising since the use of 14 weight % Surfynol 485, in the absence of any of the preferred glycol humectants of this invention, is shown hereinafter as not being effective to allow printing in a drop-on-demand printer. That is, it provides insufficient glycol or other humectant.
Other inks have been taught that use high levels of humectants, such as glycols. These humectants are particularly needed, as noted above, in such large quantities for drop-on-demand printers, since the ink is retained in a quiescent state for so long between demands. Without the high weight % of humectant, the printhead crusts over. A representative patent illustrating an ink with such properties is shown in U.S. Pat. No. 4,239,543. However, this patent makes no mention of the use of plain paper, nor does it suggest that acetylenic diols should be added to the humectants already present, for any reason. In fact, plain paper is not acceptable for such ink, due to feathering caused by the high weight % of humectant.
Finally, yet another reason mitigates against the addition of a surfactant to an ink already heavily loaded with a humectant. That is, it is difficult to find a surfactant that accomplishes control of the drop appearance on plain paper, without adversely affecting the ink's property in some other respect, such as the ink's drying time.